Electromagnetic valves provided in actuators for hydraulic pressure control in brake apparatus have been typically designed such that a differential pressure amount generated between upstream and downstream flows of the electromagnetic valve is varied. One method involves ON/OFF duty control of an electric current flowing to a solenoid of the electromagnetic valve, which achieves a differential pressure amount in accordance with a duty ratio.
Under such duty control, however, operation noise may be generated due to pulsation of a brake fluid pressure. Therefore, a new method was tried to enable linear control of the differential pressure amount generated between the upstream and downstream flows of the electromagnetic valve, in accordance with an electromagnetic force that is applied to the electromagnetic valve.
In order to enable linear control of the differential pressure amount generated between the upstream and downstream flows of the electromagnetic valve, the relationship described below must be achieved between an electromagnetic force for attracting a plunger, and the sum of a fluid force acting on a valve body as a force resisting the electromagnetic force (hereinafter referred to as a “resistance force”) and a spring force of a spring for biasing the plunger (with such sum hereinafter referred to as “fluid force+spring force” (or as a “resistance force”)).
FIG. 9 is a characteristic diagram illustrating the relationship between the electromagnetic force and fluid force+spring force. FIGS. 10A to 10C are pattern diagrams illustrating actions of the valve body in order to explain the relationship in FIG. 9.
As FIG. 10A shows, the electromagnetic force and fluid force+spring force are equal when the valve body is assumed to be at a predetermined balanced distance from a valve seat. A linear-controlled valve generating a differential pressure force in accordance with the electromagnetic force must be designed such that the valve body returns to the original balanced position. That is, the valve body must return to the original balanced position regardless of imbalances between the electromagnetic force and fluid force+spring force, which may be caused by any external disturbances, such as fluctuations in the fluid force or vehicle vibration.
Therefore, defining a balance point where the electromagnetic force and fluid force+spring force are equal as shown in FIG. 9 as a reference position, the electromagnetic force must be greater than fluid force+spring force so as to draw the valve body back toward the valve seat side when the valve body moves in a direction away from the valve seat (in a direction that opens the valve). Alternatively, fluid force+spring force must be greater than the electromagnetic force so as to draw the valve body away from the valve seat when the valve body moves in a direction approaching the valve seat (in a direction that closes the valve). In other words, the force relationships illustrated by arrows in FIGS. 10B and 10C must be achieved.
In order to achieve such relationships, an electromagnetic valve described in Published Japanese Patent Application No. SHO-61-41123 has an end face, that is, an attraction face, on a guide forming a core that is machined into a concave shape. Magnetism can thus escape over a broad area when an end of a plunger forming a movable core approaches the attraction face. As a result, changes in an attraction force with respect to a valve body stroke (an attraction force gradient) becomes gentler than a gradient of fluid force+spring force, thus attaining the above relationships.
An electromagnetic valve described in a Japanese translation of PCT International Application No. 2000-512585 has a hemispheric shape achieved by forming an end of the cylindrical valve body provided on an end of the plunger into a spherical shape. This in turn increases a fluid force gradient, thus attaining the above relationships.
In the electromagnetic valve described in Published Japanese Patent Application No. SHO-61-41123, however, complicated machining is required in order to achieve the concave-shaped attraction face of the guide forming the core. Furthermore, complicated machining is also required in order to achieve the spherical-shaped end of the cylindrical valve body, which is provided on the end of the plunger in the electromagnetic valve described in the Japanese translation of PCT International Application No. 2000-512585. Therefore, the problem of complicated machining in order to attain the above relationships arises with regards to the electromagnetic valves in both Published Japanese Patent Application No. SHO-61-41123 and the Japanese translation of PCT International Application No. 2000-512585.